Previous methods of pumping a solid state laser gain medium include side pumping with diode bars or flash lamps and longitudinally pumping the solid state laser gain medium with another laser or with laser diodes. In a typical side pumped laser, the entire solid state gain medium is pumped and the gain medium is larger (spatially) than the fundamental transverse mode volume of the laser gain medium. In order to ensure operation in the fundamental transverse mode (TEM.sub.00), an intra-cavity aperture is required. This intracavity aperture typically induces substantial loss in energy or power, and can reduce the beam quality due to the effects of diffraction around the aperture.
In diode laser end-pumping (longitudinal pumping), the beam from a group of single element cw diodes are circularized and focused down the longitudinal axis of the laser crystal such that there is a high degree of overlap between the region pumped by the diodes and the fundamental transverse mode volume of the crystal. The crystal is typically longer than the absorption depth, leading to almost complete absorption of the pump beam. This fact, combined with the high degree of overlap with the fundamental transverse mode volume in end-pumped geometries, has lead to the highest reported slope efficiency of any laser. Progress in the construction of single element diodes, however, has not yielded the high power diffraction limited beams necessary to make high power end pumped solid state lasers. Currently, semiconductor master oscillator, power amplifiers are the highest power devices available that readily lend themselves to end pumping of solid state lasers. These devices, which rely on an index guided master oscillator are currently commercially available with only 1 or 2 watts average power.
The above constraints have lead laser engineers and researchers to end-pump laser crystals using gain guided, quasi-cw laser diode bars whose peak powers now exceed 100 watts. These laser diode bars are typically 1 cm long with a 1 .mu.m junction width leading to beam divergences on the order of 10.times.45 degrees, respectively. These numbers make it very difficult to collimate and circularize the laser diode beams such that they are smaller than the fundamental transverse mode volume of the solid state laser crystal to be pumped. Also, when quasi-cw diode bars are used to end-pump, they require at least two optical elements to obtain suitable beam quality. This leads to an inefficient and less rugged delivery of the pump light beam to the laser gain medium.